1. Field of the Invention
This invention relates to a vacuum arc interrupter and, more specifically, to a vacuum arc interrupter having a bullet assembly that is structured to puncture a cupro-nickel seal cup.
2. Background Information
There is the potential for an arcing fault to occur across the power bus of a motor control center (MCC), another low voltage (LV) enclosure (e.g., an LV circuit breaker panel), other industrial enclosures containing LV power distribution components, as well as medium voltage (MV) enclosures. This is especially true when maintenance is performed on or about live power circuits. Frequently, a worker inadvertently shorts out the power bus, thereby creating an arcing fault inside the enclosure. The resulting arc blast creates an extreme hazard and could cause injury or even death. This problem is exacerbated by the fact that the enclosure doors are typically open for maintenance.
It is known to employ a spring device and piston to rapidly couple a live conductor to a grounded conductor in a vacuum arc interrupter in order to short the circuit upstream of the LV components. A vacuum arc interrupter utilizes two contacts in a vacuum chamber. One contact is fixed and the other contact is movable. The movable contact includes a stem, which is coupled to a bellows, that extends outside of the vacuum chamber. The spring is coupled to the stem and to a release device. The release device is coupled to an arc sensor in the LV or MV enclosure. The stem, and therefore the movable contact, moves from a first position at one end of the chamber to a second position at the opposite end of the chamber. One contact is coupled to the LV or MV circuit and the other contact is grounded. In operation the first position of the piston corresponds to the open position of the contacts. When an arc occurs in the LV or MV equipment, the arc sensor actuates the spring release device, thereby allowing the contacts to move into the second position and short the circuit.
Another device, that is, a device which is not a vacuum arc interrupter, for shorting a circuit included a tapered slug which is propelled by high pressure gas into a tapered set of openings extending through two bus bars and a layer of insulation. The slug is maintained in a pressure chamber coupled to a gas-generating device. When gas is rapidly introduced to the pressure chamber, the slug is propelled into the tapered opening, contacting both bus bars. Typically, one bus is coupled to a live circuit and the other bus is grounded. Thus, when the slug contacts both buses, the circuit is shorted.
These interrupters suffer from several disadvantages. For example, the prior art vacuum arc interrupters require multiple components to be maintained in the vacuum chamber. Certain components, such as the bellows, are difficult and expensive to construct. Construction of the vacuum arc interrupter could be simplified if more components could be maintained outside of the vacuum chamber. Prior art vacuum arc interrupters utilizing springs, because of their nature, do not have a means for stopping the upward motion of the movable contact. That is, the spring mechanism is structured to absorb the reactive forces caused by the contacts colliding. Certain prior art vacuum arc eliminators also include a combination of springs and shock absorbers. The use of a spring or a combination of a spring and a shock absorber reduces, but does not eliminate, the bounce which occurs when the moving component contacts the stationary component. Thus, the prior art vacuum arc interrupters do not have a mechanism for stopping the advance of the moving component.
Furthermore, with regard to the prior art utilizing a slug, the slug relied on the application of gas pressure on the piston to ensure that the piston remained in the second position. Or, if the slug moved in a downward direction and the slug was heavy, gravity provided a sufficient force to hold the slug in place. That is, this system did not include a mechanical lock to maintain the slug in the second position. Additionally, the prior art slugs have a generally flat pressure surface. Because the gas is typically introduced through a small opening, the pressure distribution on the slug pressure surface is uneven. The uneven pressure distribution prevents the slug from moving as fast as a slug where the pressure distribution is even. Another disadvantage of this device is that, where the slug is received in a conductor having a small cross-sectional area, the electromagnetic field created by the contact may by very strong.
The present invention provides a vacuum arc interrupter having a first conductor disposed within a vacuum chamber and a second conductor disposed outside the vacuum chamber. The second conductor is in electrical communication with a seal that forms a portion of a vacuum assembly that defines the vacuum chamber. The vacuum arc interrupter further includes a pressure chamber assembly having a bullet assembly that is initially spaced form the second conductor and is structured to be rapidly propelled into contact with both the fist and second conductors, thereby electrically coupling the two conductors. As such, the bullet assembly must puncture the seal prior to contacting the first conductor.
It is advantageous for the bullet assembly to cleanly contact both conductors. Accordingly, there is a disadvantage in allowing the seal to fragment as it is being punctured as resulting particles or pieces of the seal could interfere with the connection between the bullet assembly and the conductors. Thus, this invention provides for a blade tip on the bullet assembly that is structured to cleanly puncture the seal while minimizing the amount of fragmentation. This invention further provides that the seal be made from a cupro-nickel material. The cupro-nickel material is structured to be torn without a substantial amount of fragmentation.